1. Field of the Invention
The present invention relates to a noise reducing circuit in which various kinds of noises contained in the demodulated output of a radio receiver is reduced by the use of fuzzy control.
2. Prior Art
There have been a variety of circuits for reducing noise in a receiver output. FIG. 1 shows one such noise reducing circuit using a feedback control.
In FIG. 1, a noise reducing circuit 20 includes a noise attenuating circuit 21, a noise level detecting circuit 22, a control signal generating circuit 23.
The noise attenuating circuit 21 takes the form of, for example, a frequency-characteristic controlling circuit, stereo-separation controlling circuit, muting control circuit, or a combination of these. The noise detecting circuit 22 detects a noise level in the output of the noise attenuating circuit 21 to output a noise-level signal. The control signal generating circuit 23 compares the noise-level signal with a fixed reference voltage Vs so as to output a control signal whose magnitude is proportional to the difference between the noise-level signal and the reference voltage Vs. The control signal is fed back to the noise attenuating circuit 21. Thus, the feedback control effectively reduces the noise in the receiver output.
In general, the field intensity of an input to a receiver varies over a wide range. Thus, as shown in FIG. 3B, when the field intensity varies beyond the noise control range of the noise reduction circuit, the noise level in the receiver output rapidly increases. A noise reducing circuit in an FM receiver such as the above-described frequency characteristic controlling circuit, stereo-separation controlling circuit, and muting control circuit has only a narrow range of control, e.g., about 20 dB for the stereo-separation controlling circuit and about several decibels for the frequency characteristic controlling circuit. This does not allow sufficient noise-reducing effect over a wide range of field intensity.